1. Field of the Invention
This invention relates to a structure for lifting one axle of a tandem load-bearing axle assembly when the load does not require the additional axle for support on the roadway.
2. Description of the Prior Art
The use of auxiliary axles on trucks for providing additional load-bearing wheels is well known in the art and additionally, it is common to hydraulically lift the additional wheels from the road surface when the load is light and the additional load-bearing wheels are not required to meet the axle or load limits. The prior art discloses axles which may be lifted by fluid means acting through the shock dampening support means for the auxiliary axle and other auxiliary road wheel assemblies and lift mechanisms are known in the patent art.
Of the known patents utilizing an auxiliary road wheel structure, in U.S.A. Pat. No. 2,902,289, issued Sept. 1, 1959 to O. G. North. This patent discloses the use of independent wheel structures mounted one on each side of a truck frame. Each independent wheel structure comprises a axle for supporting dual tires and a hydraulic lift mechanism using a bell-crank for forcing the wheels into engagement with the roadway and for lifting the wheels from the roadway. In this structure the hydraulic cylinder is mounted between the bell-crank and a compression spring structure for absorbing road shocks when the wheels are forced into engagement with the roadway. In each structure illustrated in this patent the auxiliary wheels are positioned forwardly of the conventional axle on either the tractor 10 or on the trailer 11. In such a structure it is noted that the cylinders must be active at all times to maintain the wheels in engagement or to lift the wheels. In contrast, in the present structure the fluid pressure is used only when carrying the wheels in a raised position. The special wheel structures illustrated do not anticipate the present invention which is directed to a device for use with trucks having tandem axle structures and which is added to lift the trailing axle to avoid the additional wear on the tires of the auxiliary axle when the load does not require the extra load-bearing wheels.
It should be further pointed out that there are other teachings of lifting wheels which are normally support wheels, out of engagement with a road bed, such as in the structures illustrated in U.S.A. Pat. No. 2,953,390, issued Sept. 20, 1960, to J. H. Hogstrom and U.S.A. Pat. No. 3,235,285, issued Sept. 15, 1966 to P. Tenebaum et al. These patents fail however to anticipate the teaching of the present invention.
The patent to Hogstrom discloses a trailer construction having a tandem axle structure to support the trailer. When the load is delivered the crowned axle support members permit one set of wheels to be lifted and held by the tow chains which can be passed through a hook or loop 65. Such a structure would not be suitable on more conventional tandem assemblies. The patent to Tenebaum et al dislcoses a single axle wheeled unit utilized to raise the conventional tandem wheels from the ground to adjust the height of a trailer to the height of a dock. These structures fail to anticipate the structure of the present invention.